The invention further relates to an apparatus for measuring paper color, the apparatus comprising a measuring device which is arranged to measure paper color from the surface of a paper roll on a winder during, the winding of the paper, and to means for keeping the angle between the measuring device and the paper roll constant.
The article "An on-line sensor for the measurement of paper colour" in the publication "Appita Vol. 44 No. 2" describes a method and apparatus in which paper color is measured on the winder. In this case the caliper of the paper to be measured and its background color cannot affect the measurement result because a thick layer of paper is very quickly formed on the winder, and the paper provides for the color measurement an even backing which also has the same color as the sample to be measured. The measuring device is immovably arranged at a distance from the winder, therefore background light easily interferes too much with the measurement result, and the measuring device is also difficult to calibrate. As the circumference of the paper roll grows, distance to the roll surface must be measured and the measurement angle adjusted on the basis of the measurement, which makes the apparatus complex and cumbersome. Further, the growing circumference of the paper roll changes the distance between the measuring device and the paper surface, which in turn changes the measurement geometry of the apparatus, i.e., the focusing of a light beam directed from the measurement apparatus on the surface of the paper, whereby the measurement provided is inaccurate.
An object of the invention is to provide a method and apparatus allowing the above mentioned drawbacks to be avoided.